Being one of ω3 fatty acids, eicosapentaenoic acid (EPA; (5Z,8Z,11Z,14Z,17Z)-icosa-5,8,11,14,17-pentaenoic acid; hereinafter, the simple designation of “eicosapentaenoic acid” or “EPA” means this substance) is known as a component that shows an anti-arteriosclerosis action, a platelet aggregation suppressing action, a blood lipid lowering action, and so on. Being an ester form of eicosapentaenoic acid, ethyl eicosapentaenoate (also described as “EPA-E” or “ethyl icosapentate”) is sold as a health food, switch OTC, a pharmaceutical, and so on.
On account of these outstanding functions of EPA, many methods to produce EPA or EPA-E have also been reported. For instance, it is known that when a mixture obtained from a natural oil or fat is subjected to rectification with three or more distillation columns under high vacuum, a fraction that substantially consists of only a C20 fatty acid or an ester thereof is obtained and may be further refined to give a high-purity EPA or an ester form thereof (see JP HS-222392A).
On the other hand, it is known that the refined EPA-E also contains ethyl esters of fatty acids other than EPA derived from fish oil as the starting material, and impurities that are generated in the refining process. Exemplary fatty acids other than EPA derived from fish oil as the starting material include arachidonic acid that is classified as an ω6 fatty acid and considered to be a substance that is unfavorable to cardiovascular events, and saturated fatty acids. Known as impurities that are generated in the refining process are fatty acids that result from thermal denaturation in the refining process and in which the five cis double bonds of EPA have been partially isomerized to trans (e.g. European Journal of Lipid Science and Technology, 108 (2006) 589-597, “Geometrical isomerization of eicosapentaenoic and docosahexaenoic acid at high temperatures”; JAOCS, 66 (1989) 1822-1830, “Eicosapentaenoic acid geometrical isomer artifacts in heated fish oil esters”).